Phase shift components find many uses in electronic circuits. A typical phased array antenna may have several thousand radiating elements with a phase shifter for every antenna element. Ferrite phase shifters have gained popularity due to their weight, size and operational speed characteristics. However, unit cost and complexity of ferrite phase shifters have prevented their wide spread use. PIN diode phase shifters are cheaper than ferrite phase shifters, but exhibit an excessive insertion loss which limits their utility in antenna arrays. Phase shifters that employ ferroelectric materials have the potential to provide much better performance than ferrite and PIN diode phase shifters due to their higher power handling capacity, lower required drive powers and wide range of temperatures of operation.
The discovery of the ferroelectric barium titanate opened the present era of ceramic dielectrics. In such ferroelectric dielectrics, pre-existing electric dipoles, whose presence in the material is predictable from crystal symmetry, interact to spontaneously polarize sub-volumes. A ferroelectric crystal of barium-titanate generally consists of localized domains and within each domain the polarization of all unit cells is nearly parallel. Adjacent domains have polarizations in different directions and the net polarization of the ferroelectric crystal is the vector sum of all domain polarizations.
The total dipole moment of a ferroelectric crystal may be changed (i) by the movement of walls between the domains, or (ii) by nucleation of new domains. When an external electric field is applied, the domains are oriented. The effect is to increase the component of polarization in the field direction. If the applied field is lifted, some of the regions that were oriented retain the new orientation; so that when a field is applied in an opposite direction, the orientation does not follow the original path in the curve. More specifically, the crystal exhibits a hysteresis which equates to a loss function for electrical signals that propagate therethrough. Such hysteresis action occurs when the ferroelectric crystal is operated below its Curie point temperature. Above the Curie point temperature, the crystal is both isotropic and paraelectric in that it does not exhibit the hysteretic loss function. In order to reduce the hysteresis effect, others in the prior art have added dopants to the crystalline matrix to, in essence, provide a "lubricating" function at the domain boundaries which reduces the remanent polarization upon a retrace of the hysteresis curve.
Barium titanate and barium titanate-based ceramics exhibit high dielectric constants (on the order of 2,000 or more). By application of a variable voltage bias across a barium titanate crystal, substantial "tunability" (variation of the dielectric constant) can be achieved. Nevertheless, as a result of the high dielectric constant values, the use of barium titanate materials as phase shifters in microwave applications has been limited (due to a high level of mismatch with the material into which the electric waves are coupled, e.g. air). Further, because the Curie temperature of barium titanate is approximately 120.degree. C., operation of barium titanate-based ceramics at ambient assures that they operate in the region where they exhibit the hysteresis effect-and thus exhibit the loss function associated therewith.
More recently, it has been found that the inclusion of various amounts of lead, calcium and strontium can substantially modify the Curie temperature of a barium titanate ceramic. In FIG. 1, a plot of Curie temperature versus mole percentage additions of isovalent additives lead, calcium and strontium is plotted. It is to be noted that only a strontium additive enables a substantial lowering of the Curie temperature to a level that is both at and below normal ambient operating temperatures. As a result, barium strontium titanate (BST) ceramics are now being investigated in regards to various electronic applications.
BST ceramics exhibit a number of attributes which tend to make them useful for microwave phase shift applications. For instance, they exhibit a large variation of dielectric constant with changes in DC bias fields; low loss tangents over a range of operating DC bias voltages; insensitivity of dielectric properties to changes in environmental conditions; and are high reproducible. Nevertheless, they still exhibit very high dielectric constants which create substantial mismatches in phase shift environments.
Accordingly, it is an object of this invention to provide improved ferroelectric dielectrics that are suitable for use with electronic applications.
It is another object of this invention to provide improved BST dielectrics which exhibit low dielectric constants.
It is yet another object of this invention to provide low dielectric BST materials which retain a substantial tunability characteristic.
It is yet another object of this invention to provide improved BST materials that exhibit both low dielectric constants and operate in the paraelectric region at ambient temperatures.